This invention relates in general to X-ray tomography and, more particularly, to methodology for processing image data with respect to X-ray tomography.
In general, the process of generating an image with computer X-ray tomography involves exposing a test object to X-rays from an X-ray source. The test object is placed between the X-ray source and a detector array which samples the radiation passing through the test object. Those portions of the object which are more dense pass less radiation through to the detector array than do portions of the test object which are less dense. The object may be rotated to provide a plurality of different views, or alternatively, the X-ray source and detector array may be rotated together to provide such different views. Tomography can be used to generate either two dimensional (2D) or three dimensional (3D) images.
In more detail, the conventional 2D tomography method involves the step of orienting the test object in a first position and exposing the test object to the X-ray source. The resultant detector data is acquired in the form of a linear array of samples. The test object is then rotated and another linear array of samples is obtained. Several linear arrays of samples are obtained in this manner and are used to fill a Radon space. Once the Radon space is filled, an inversion is performed on the Radon space to obtain a 2D image.
A 3D image can be created by a similar approach wherein a planar array of samples is taken at each rotation of the test object. The resultant planar arrays of samples are used to fill a Radon space and, once filled, the Radon space is inverted to obtain a 3D image. One of several known computed tomography algorithms are employed to provide this inversion to construct the final image. See, for example, U.S. Pat. Nos. 5,068,882 and 5,073,910 which describe Radon space conversion algorithms.
To obtain relatively high resolution images with the conventional techniques described above, it is often necessary to employ very fine angular steps while the test object in rotated. This results in a large number of linear arrays of samples or planar arrays of samples. Unfortunately, this also results in relatively high exposure of the test object to X-rays since so many arrays of samples must be taken.
Undesired artifacts in the final image are known to result if the number of sample arrays is too small or the angular steps between sample arrays are too coarse. To avoid these artifacts, the usual approach is to make the angular steps between sample arrays (detector angles or test object angles) relatively fine or to interpolate in Radon or Fourier space. Such interpolation in Radon space is computationally intensive and fraught with difficulties due to relatively low correlation of adjacent samples in Radon space.
Unfortunately, the problem of insufficient Radon filling is often ignored resulting in undesired artifacts appearing in the resultant image. Even with a relatively high number of samples taken at various finely stepped detector test object angles, image artifacts sometimes result.